text:
What does two 'high-low' chimes in the cabin of a 737-800 mean?
Recently on a trip to Orlando on Sun Country, I came across a cabin chime that I'd not heard before. It sounded like two 'high-low chimes' in a row, so basically equivalent to someone ringing a doorbell twice.
I know that one 'high-low' means that the FAs are speaking to each other through the phone in the aircraft, but I've never heard it twice in rapid succession.
Are all 737-800s equipped with these chimes? What do they mean?

As far as I know, the chimes work the same on all 737 NG (and Classic) versions, although as was mentioned in comments, how they are employed will vary from airline to airline.
The two tone "high-low" chime described in the OP is the "crew call" system. The pilots have a button, the forward F/A station has one, and the aft F/A station has one.  Regardless of which button is pushed, the "hi-low" chime is the same.  In many, though not all, cases, this chime is a signal to the F/A's to answer the interphone.  If the pilots press their button, they're asking to speak to the F/A's; if one F/A station presses the button, they're asking to speak to the other F/A station.  (They can also press a different button that calls the cockpit, but passengers won't hear that chime. In the cockpit, we hear that "call" as the two-tone chime; we don't hear the chimes in the cabin unless we're listening to PA audio.)
At one carrier, a single high-low chime may mean "answer the interphone," while at another carrier, one chime might be used to mean "secure the cabin" while two chimes is "answer the interphone," three chimes is "answer the interphone - urgent," and so on.
You'll also hear chimes when the F/A Call Button above a passenger row is pressed; this is a distinct chime (a single, relatively high note).  There is also a chime when the Fasten Seatbelt sign is turned on or off, this is again distinctive, a single lower note.
Note that you won't necessarily hear the chimes when the interphone is in use; it's simply a "Hey!" call to the Flight Attendants in the cabin.  While it's often followed by a conversation on the interphone, they two are separate systems and one can be used without the other.

So basically you are saying that it varies by airline?

What an awesome answer! Also, if one F/A doesn't pick up the interphone immediately, might that cause the chime to ring more than once?
@Ralph J Are you a pilot?

If nobody answers the interphone, it's assumed that they're busy -- middle of the airplane serving drinks or whatever.  If they don't answer in a minute or so, I'd assume that nobody heard the first chime & try again, but that's pretty uncommon.  Yes, I've flown the 737.

Ralf has said it in a great way.
The chimes are part of the cabin / crew intercom. What they mean depends. The carrier I work for, uses a single chime to advise the cabin crew that they have to secure the cabin. Typical calls are indicated by the two high-low chimes you have described. Whether you hear them or not depends. In many cases - especially in smaller aircraft - using the chime to call is not required, since the cabin is relatively small and the crew is close to each other.
In this case a chime can only be heard, when the cockpit calls the cabin. In exchange we can hear it, when the cabin calls us in the cockpit. Other than that, pilots do not hear what is happening in the cabin, unless they link themselves into the intercom of the cabin crew, which is - at least by us - rarely done, as there is no reason.
Cheers


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What exactly is an "engine surge" in a jet engine, and what causes it?
Surge suggests that something speeds up, yet I read elsewhere that in fact in jet engines, an engine surge is actually a compressor stall.
Whether that is correct or not, what exactly is doing the surging in the case of an engine surge?
What kind of fault within the engine or conditions of its environment causes an engine surge?

Compressor rotating stall and engine surge are also explained in If the intake of a running jet engine is blocked for some short duration what would be the sequence of events following it?

The surge is the increase in rpm of a stalled compressor.  The compressor blades are airfoils not unlike propellers and wings. They will stall if airflow is not maintained at the proper angle of attack.
Interruption of airflow in jets can be caused by abrupt maneuvering, ingestion of foreign matter, or excessive application of throttle.  Much has been done to eliminate this issue in modern jets, but the nature of the design still makes it possible. If air flow is interrupted to the compressor, the jet can literally backfire, creating a loud "bang".  A large yellow flame may also be seen coming from the back of the jet as the fuel/air mixture is now excessively rich due to insufficient air.
If the engine is not damaged, it can be re-started the same way we unstall a wing, by re-establishing proper airflow angle of attack.  This is done by lowering pressure BEHIND the compressor.  Throttle is reduced. Provided the engine is not damaged, throttle can be carefully restored.

Sometimes the backfire is bad enough to get flashes of flame at the front.  Also note you can also get surge with centrifugal compressors if you choke off enough flow although they are way less sensitive.  Basically like blocking a vacuum cleaner.  Moderate stalling and surging will produce an oscillating cycle that comes through the airframe as a "rumble" sensation.  In the old days when suck-in doors were first introduced to provide extra air during flow disruptions, they were called "de-rumble doors".

Modern FADECs usually implement the recovery procedure automatically. When they detect engine surge, they restrict fuel flow and gradually increase it back and only if the engine surges again generate a warning.

So basically, it's an aerodynamic stall within the engine, as opposed to a stalling of the engine like you'd see in a ground vehicle?

Correct.  Amazing how they got that right.  Instruments would read out a "surge" in rpm as the compressor unloads "like blocking a vacuum cleaner (perfect)". Can also be considered a "surge" forward of higher pressure air (and flame) as stalled compressor can not maintain proper airflow.  Jets have a continuous combustion process, which helps account for their huge amount of power for their weight compared with piston engines.

Once I was riding in an Air France 747 that had a surge event at the start of the takeoff roll.  As the thrust came up, there was a boop-boop-boop-boop-booop BOOM sound, with a flash of light (it was at night), then a spool down and we taxied back to the gate.

@John K Did they keep you all on the same plane or did you have to switch?  (That's a lot of luggage to swap!)

It was quite late, just before curfew, and it was transatlantic trip, so the flight was cancelled and resumed in the morning.  It was my home airport so I just went home.  I always traveled on business with carry-on baggage only, so I don't know if they kept everybody's checked bags.  I would imagine they did.

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text:
Can a pulse jet be used on a light GA aircraft?
I have an idea for designing a light, cheap and fast 2-seated private airplane... I was thinking about using a pulse jet engine which is simple to design and build, and is cheap too.
Noise is not an important concern in my region. The only things that I'm wondering about are efficiency, fuel consumption (range), vibrations and cooling.
Is it feasible to use pulse jet instead of regular engines?
Is the pulse jet functional and operational?

In principle yes. But some details will turn most potential operators off.
Noise is the obvious first one. You might not mind, but cockpit noise and vibration in operation will certainly put an unacceptable stress on the pilot. Just think how you will communicate with ATC or with a co-pilot. Also, your choice of airports to operate from will be rather restricted.
Vibration also means that you need to ruggedise the airframe. This will certainly eat up some of the mass benefits of the pulse jet, but is hard to quantify.
Now fuel consumption: The AS-014 would consume 350 g of fuel per hour for every Newton of thrust. Compare this to the 60 g/Nh of a modern turbofan. The additional fuel will eat up the remaining mass advantage once you desire to fly for more than just a few minutes.
Next will be take-off distance. Unless you use a catapult, you will need a long runway. The pulse jet has limited static thrust, so your initial acceleration will be poor. At the high end of the speed envelope, though, thrust should be plentiful.
Cooling, however, is no problem. Just expose the pulse jet to the elements and you are all set.
To summarise: Yes, a pulsejet is a possible power source for a GA airplane, but is a poor choice for a multitude of reasons. It makes more sense in an unmanned, one-way application.

Yes.  "unmanned, one-way application" for pulsejets goes back to the 1940s...

This question is pretty close.
And you should read here as well as here
Generally pulse jets are not great at low speeds (and can be hard to start when stationary). These are key aspects of small GA planes. The noise may not be an issue for you locally (in terms of noise abatements) but it may be an issue for the occupants of the aircraft.
In short it is possible (physically) but not really practical.

While the pulsejet powered Kawanishi Baika from the WW2 was really far from the GA, from the picture it looks like it was a relatively small plane of very comparable size, capable of taking up under own power, manned. While none were eventually built, seems that the idea was accepted rather seriously at the time.  (Its sister ship is the Fieseler Fi 103R, of which 175 were built and a few were test-flown.  But they launched by dropping from a bomber, which avoided the ground-launch disadvantages noted in Peter Kämpf's answer.  Again, not practical for light GA.)

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text:
How can business jets use slot coordinated airports?
Many capacity-limited airports in Europe are so-called coordinated airports, meaning an airline must have a slot allocated to land at this airport. In Germany for example, these are FRA, DUS, HAM, MUC, STR, TXL and SXF. Often times, these airports are also used by general aviation aircraft, i.e. business jets, ambulance flights, government flights, etc.
Do airports "reserve" a given number of slots for such flights? Or are those flights exempted from the slot system?
I'm mostly interested in European regulations.

Related (slot coordination in US vs EU): What are the practical impacts of airports in the US being mostly IATA Level 2?

Depends on the particular airport, but I will take The Netherlands (Which has three slot-coördinated airports, including EHAM, EHRD and EHEH) as an example here.
According to the ACNL all flights to and from a coordinated airport can only be executed when an air carrier or any other aircraft operator has been allocated an airport slot. According to Article 2, paragraph g, of the Regulation as state flights, emergency landings and humanitarian flights are exempted. General aviation is therefore subject to slot coordination (at EHAM and EHRD). However, according to the LOC GA AMS, helicopter and GA propeller-driven aircraft under 5700kg MTOW are exempted from the slot system.
In conclusion: Regulations depends on country and airport. You should carefully read them before starting your flight to and/or from a coordinated airport.
A detailed PDF on how to request a slot can be found here.
GA slots for Europe can be requested through e-Airportslots

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Is the standby altimeter required for day VFR flight in a C172 Nav III?
I am very confused about whether the standby altimeter is required for day VFR flight: the KOEL (Kinds of Operations Equipment List) says “0” for day VFR but “1” for day IFR, while the equipment list has it as “R” (required).

You don't need the standby altimeter for VFR (day or night) but you do need it for IFR.
The Comprehensive Equipment List (section 6 in the POH) lists all the equipment required by the type certificate:

Required items or equipment for FAA certification (14 CFR Part 23
  or Part 91)

But, even if a piece of equipment is required by the type certificate that doesn't mean it's required for a specific flight. The Kinds of Operations Equipment List (section 2 in the POH) lists the equipment that's required for airworthiness in day and night VFR and IFR:

identifies the equipment required to be operational for airplane airworthiness
  in the listed kind of operations.

14 CFR 91.7 says that the aircraft must be "in an airworthy condition" to fly, and that's what the KOEL defines.
The reason that the standby altimeter is marked as required in the Comprehensive Equipment List is that the C172S was type certified under the old part 23 rules. 23.1311 required IFR-certified aircraft that use electronic displays - like the G1000 in the NAV III - to have a standby altimeter, either mechanical or electrically isolated from the main one (emphasis mine):

For certification for Instrument Flight Rules (IFR) operations, have
  an independent magnetic direction indicator and either an independent
  secondary mechanical altimeter, airspeed indicator, and attitude
  instrument or an electronic display parameters for the altitude,
  airspeed, and attitude that are independent from the airplane's
  primary electrical power system

In summary:

The standby altimeter is marked as required in the Comprehensive Equipment List because it's an IFR certification requirement
Operationally, the standby altimeter is only required for IFR flight, per the KOEL

it is required equipment under 14 CFR §§91 under the Operational Equipment List according (OEL) for the Type Certification, according to the POH.  Flight operations may not be conducted without a special airworthiness waiver issued by the local FAA FSDO and only for the purposes of ferrying the airplane to a facility where the maintenance can take place. See §§91.213.


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How does auto landing work?
Some aircraft are equiped with auto landing to assist on zero visibility and ceiling land. Actually, how does this system work?

Possible duplicate of How does ILS (Instrument landing system) work?

@fooot Why is this a dupe? The question is how autoland works, not how the ILS works. The ILS is used by the autoland system, but they aren't the same thing.

The question isn't clear which part it's asking about, and ILS is the guidance system being used.

@fooot True, but it requires a radar altimeter too, as far as I know (which isn't much). Anyway, I think you're right that the question isn't clear but in that case I think we should ask for clarification and/or close as "unclear what you're asking" rather than closing as a dupe. That's my two cents anyway.

The ILS question doesn't provide details about the autoland requirements (e.g. the precision required, the redundancy). Also, the way the flare out is executed or the ground roll are interesting. I think these elements deserve a dedicated answer.

@user12207 You may edit your question to precise which part of the landing (nav to approach path, approach guidance, flare maneuvre, landing roll-out, more than one of those part) you are interresting in.

Why on Earth was this flagged as belonging on another SE??

Autoland is an autopilot feature which allows the autopilot to control the aircraft all the way to touch-down and even a bit further.
Basic idea is that the autopilot is accurate enough to keep the aircraft on the correct approach path. This is achieved by (on top of really good software engineering) with at least 2 autopilot/flight director calculators. One of the calculators is the one in control, anfd the other one is monitoring the first one. In case of disagreement between the systems, a fault is announced and the approach must be aborted (fail-passive system).
There are also fail-active systems that require 3 autopilot channels. These are capable of continuing the approach even if one system gets faulted.
Some systems have a roll-out guidance to keep the aircraft aligned on the runway centerline after touch-down during braking.
Functionality is the same as with autopilot. The calculator gives commands to the control surfaces and thus steers the aircraft. In addition a radio altimeter gives accurate altitude information to enble flare and thrust reduction.


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How can I check that the GPS receiver is sending data to the transponder?
I fly an ultralight aircraft. The GPS is a puck-type device from which the TX line runs to the mode-s transponder. The transponder does not readily show if it is receiving position data. The data is NMEA RMC sentences only, serial 4800 baud 1 message / second.
Is there any way to ascertain as part of the preflight check that the GPS is alive and sending some data (how correct is besides the point, for now)? I'm thinking along the lines of connecting an LED in parallel on the TX line which will flash to show data traffic if I press a test switch. Any ideas? Thx

You could buy a $25 usb TV receiver dongle and have it read the mode s data, to avoid having to modify your equipment. But then you must also always bring a laptop.

An arduino might be a better choice than a laptop for that application.

Silly of me not to include the data type and format. Added, thx.

Even if the GPS is outputting NMEA sentences, that doesn't necessarily mean the transponder is receiving them. There could be a break in the connection at the transponder, and your device on the wire before that wouldn't catch that particular failure.

What exact GPS puck/transponder do you have?   Could you provide serial numbers?   We need to know at what voltage the tx line is running on and whether it idles high or low (whether the voltage on the line is pulled high or low when there is no traffic).   If you provide this I can help you out.

Just curious, is the transponder sending ads-b data?

Wow. The puck is a GlobalSat BR355 (serial) and runs on 5V. Yes, the transponder sends position data from the GPS as ADS-B out. The transponder is a Funke TRT800H. Big thanks!

Have you tried seeing if you show up on FR24? If you're within range of a receiver it should show. You might have to get up in the air to hit a receiver.

Your puck (GlobalSat BR355) runs on 5v and the UART line idles high (it is pulled high when there is no traffic).   The first thing you need to do is find the power lines that go to the gps puck -- connect wires to these lines (5v and gnd) and run them to the spot where you want the button/led.   Then make this circuit:

The value for R1 should be 150 ohms (which will yield under 20ma of current through the LED, assuming your led has a voltage drop of 2.1v (which the ones in the link I provided do have))

Here is a link to a transistor that will work for Q1:
https://www.digikey.com/product-detail/en/central-semiconductor-corp/2N4403/2N4403CS-ND/4806893
Here is a resistor that will work for R1:
https://www.digikey.com/product-detail/en/stackpole-electronics-inc/CF14JT150R/CF14JT150RCT-ND/1830603
And here is a link to a selection of LEDs you can use (all with voltage drop of 2.1v; Any of these should work; just choose a color and form factor that works for your application):
https://www.digikey.com/products/en/optoelectronics/led-indication-discrete/105?k=led&k=&pkeyword=led&pv16=2&FV=1140050%2C4fc0015%2C1f140000%2Cffe00069%2C87c001d&mnonly=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25
Here is a link to a selection of pushbuttons that should work -- just choose a form factor you like (look at the datasheet for the ones with 4 pins -- these are just two pairs of pins (each pair is connected) for mounting ease -- two pins can be harder to mount on a circuit board):
https://www.digikey.com/products/en/switches/pushbutton-switches/199?k=pushbutton&k=&pkeyword=pushbutton&pv130=7&FV=1140050%2C2dc0017%2C3ac000e%2C1f140000%2Cffe000c7&mnonly=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

I personally recommend you use something like this for the button:
https://www.digikey.com/product-detail/en/e-switch/KS-01Q-01/EG4791-ND/2116270
For this button here is a picture and a schematic (as you can see, the pins on either side of the flat side of the button are connected:

And something like this for the led:
https://www.digikey.com/product-detail/en/cree-inc/C503B-RAN-CZ0C0AA1/C503B-RAN-CZ0C0AA1-ND/6561758

Just wire these components up according to the picture and solder them together -- I'll leave how to mount the switch and the LED up to you but make sure once you are done you cover all the exposed wire with glue and/or electrical tape and/or liquid electrical tape so they don't accidentally short out.   Also make sure no exposed wires within the circuit are touching where they shouldn't touch (especially make sure you separate the pins of the transistor because if they accidentally touch the circuit will not work and you may damage your gps receiver -- long story short be careful in your wiring and make sure nothing is touching that shouldn't.

Thx Murey! The puck runs on 5V, so this would mean a higher R1? I will take precautions. I put details of the puck and transponder in an earlier comment. (I'm still learning to use the forum)

By the way I made two changes -- one is that I realized the button must be inline with the base of the transistor for the circuit to work properly (so you can't move it around from where it is in the schematic now) and I changed the resistor value to 150 ohms just to put less stress on the LED (I updated the link to the resistor).


text:
Exactly where can a stowaway hide in a wheel well?
My question is exactly where a person could contort their body so as not to be mangled by the gear retraction system and very limited space. Some lived and died in the nose well and as I remember it, most aircraft at least provide half the nose wheel diameter (for several feet) to the nose strut (between strut and gear doors). The nose doors suddenly opening would explain why they fell to their death.
Presumably most people attempt to stow in the main gear and I just don't understand how?
WIKI lists approximately 150 known wheel stowaways. A cursory review shows about 20% percent lived through the ordeal. Most die of hypothermia, hypoxia, trauma from falling on the ground or in the air, but a few are crushed or mangled. Despite my knowledge as a pilot and A&P, I could not make an educated guess what a person would need to do as the gear retracted to keep from being crushed.
Medical issues are not part of this question and were covered by WIKI and reviewed in a  related question that asked "How can a stowaway hide in the undercarriage?" on SE. No precise aircraft volumetric space capability was given or the "aerobatics" (I could not resist) needed as the gear retracted.
I am particularly impressed that stowaways survived DC-3, DC-8, and most surprising is the B737; these are all aircraft I have worked on and can not imagine how it is done. No one listed attempted B757 but there is a somewhat spacious 12-18in in front of the retracted main gear for cables, fuel lines and valves that could possibly allow a human space.

That's funny. I had never heard about that. How many stamps would it take to send me to Australia?

Well, considering that at least the DC-3 is unpressurized, at least oxygen supply should be less of a concern than on modern jets. Its service ceiling of 5,000 m might be problematic, but not necessarily lethal.

But the DC-3 (and Beech 18) gear folds back like a scissor (two struts and crossbar) directly behind the firewall (the tire is only about 30in dia) - not anything I want to be holding on to... it retract/extends VERY slow so at least you aren't likely to fall out.

@MichaelKjörling Airliner cargo holds of today are generally inside the pressure vessel. Living things are transported inside them regularly (and they usually survive.) Even if they didn't transport living things, who would want their baggage frozen at -40 C when it arrived? But, of course, the primary reason is just that it's easiest to design the aircraft fuselage itself to be the pressure vessel.

From my experience, cargo holds are not accessible to a stowaway on foot. They are too high and the doors are only open during loading. There is always a baggage handler inside that would see someone, and the doors are immediately closed when loading is complete. All current airliners in the western world connect the cargo and cabin through vents near the cabin wall to prevent the floor buckling during explosive decompression.

For the 737, I think this rather sad photo provides one answer:

(Source)

The body of a stowaway was found in a Kenya Airways flight from Kinshasa in the DRC on Sunday.
  The body was found after the plane landed at the JKIA, preliminary reports indicating the person froze to death.

And, as you can see in this photo, the ledge there (on the left forward side of the main wheel well) is indeed just wide enough for a slim person to contort themselves into:

(Source)
But, as you can see at the end of this video, there certainly isn't a lot of room for the stowaway's legs.

Wow, this photo really drills home how mechanically complex aircraft actually are.

@BradleyUffner and how fragile. Imagine anything getting in there with some velocity, or the tyre bursting when retracted...


text:
Is there a rule of thumb for spacing between jet engines?
While viewing this week's press release for the taxi test of the Stratolaunch I noticed how closely the engines seemed to be spaced. It would seem the ideal position would be near the fuselage to minimize asymmetric thrust.
The B-52 places engines directly next to each other, while the 747 spaces them far apart. Is there a rule of thumb for ideal engine spacing?
[This question deliberately excludes propeller aircraft due to the many mechanical and aerodynamic differences.]

The Stratolaunch engines are approximately one engine diameter apart and very close to the fuselage.

The 747 has much larger spacing between engines (~3 engine diameters) and further from the fuselage.

Engine spacing on the Antonov 225 compared to the 747 is much closer to fuselage but about the same spacing.

Part of the constraint was they had to get three engines on one wing, because the center section is reserved for the space ship.

The B52 has two engines in the same nacelle, so that would seem to be a logical minimum.

@zeta-band: They couldn't have put them on top of the center section (given that the rocket goes under there)?  Or spaced the engines further apart on the outer sections?

@selectstriker2: The B-52's engines are low-bypass JT3Ds; trying to do that with the high-bypass engines used on the Stratolaunch (or, for that matter, on the 747 and An-225 used as examples) might pose problems.

Placing the engines farther out is mainly done for bending relief in the wing. Given that the Stratolaunch needs to keep its center wing clear of fuselages and engine nacelles, the engines are out enough already. Moving them farther out would create more thrust asymmetry once one engine fails.
Airliners with a central fuselage are different. We have an answer here discussing optimum engine placement. Why they should not be combined in one nacelle is explained here.


text:
What is the origin of the word 'Type-Certificate'?
As per the definitions of Type Approval and Type Certificate, Type Approval is more related to design approval or certificate of conformity.

A type certificate (TC) is issued to signify the airworthiness of the
approved design or "type" of an aircraft to be manufactured.
Type approval or certificate of conformity is granted to a product
that meets a minimum set of regulatory, technical and safety
requirements

I have no idea why the very generic word "Type" is used to denote something totally unrelated to its meaning. What is the origin of this usage?

A manufacturer comes out with a new type of aircraft ("type" = "kind" = "model"), and it needs to be approved or certified.

I don't think the word type is a misnomer since one definition is, "a category of people or things having common characteristics". We say types of vegetables, types of animals, etc. (in common language not biology). A four-engine 707 is a different type of airplane than a three-engine 727 with T-tail. Although In the case of aviation it seems to be more about commonality in flying characteristics. A 737 with larger engines that changes the handling is a different type, unless as in the case of 737 MAX software is used to remove the difference in handling, then it is considered the same type.

@JohnK, but approval is a verb, certificate is a noun.  The certificate just documents approval, right?

@MichaelHall Actually I think I'm a little out of date. Canada has adopted "certificate" now.  Mods in Canada used to be called STAs, A for Approval, but now when I look it up it says "Certificate". I've been out of the loop for a while. There was a harmonization project some years back to better align CARs and FARs.

The word "type" has more than definition.
For example, it can also mean what you did on your keyboard to post this question...
However, in this context it is defined differently and in a manner that is entirely consistent with other uses of the word.
14 CFR defines type in Chapter 1, Subchapter A, Part 1:

(1) As used with respect to the certification, ratings, privileges, and limitations of airmen, means a specific make and basic model of aircraft, including modifications thereto that do not change its handling or flight characteristics. Examples include: DC–7, 1049, and F–27; and

(2) As used with respect to the certification of aircraft, means those aircraft which are similar in design. Examples include: DC–7 and DC–7C; 1049G and 1049H; and F–27 and F–27F.

(3) As used with respect to the certification of aircraft engines means those engines which are similar in design. For example, JT8D and JT8D–7 are engines of the same type, and JT9D–3A and JT9D–7 are engines of the same type.


text:
How do I calculate the landing distance in a landing chart with tailwind?
With the following chart I can calculate the landing distance with the headwind, gross weight and altitude. The charts uses headwind component, but how can I calculate the landing distance required if an aircraft is landing with tailwind? There is this rule of 50% for headwind and 150% for tailwind but how do i apply this with this chart?
Thanks in advance

To the extent it works at all, you find the distance with no headwind, and with N knots of headwind. Subtract to get the difference. Then add that difference to the distance needed with no headwind to get a rough notion of the distance needed with N knots of tailwind. Then you probably want to add at least a 20% margin for safety.

Simply put, do not land with tailwinds.

You can’t use that chart with tailwinds - not without an expansion of the graph to include that information.  Without more information from the OEM on landing distance in a tailwind I would not risk my life on a calculation derived from that chart.

You can see that the lines of equal landing distance over wind speed are equally spaced. This means that landing distance changes linearly with wind speed. Therefore, you can (within limits) extrapolate into the tailwind range. But strictly speaking this chart does not give information about landing distances in a tailwind.
There is no magical change once wind changes to a tailwind, so an extrapolation will give you good answers for low to moderate wind speeds. However, @Carlo is right: This chart does not cover tailwind landings, so you will be outside of the performance figures established by the manufacturer.


text:
Does the dew point affect density altitude?
I am looking for the most accurate formula to calculate the Density Altitude (DA) to use it for calculating aircraft performances during landing.
Now I found the following formula:
DA = PA + [120*(OAT - ISA temperatures at given altitude)]
PA = ((standard pressure - given pressure)*30) + field elevation
My question is: Why is the dew point not included in this formula and what influence does it have on the density altitude? What is the correct formula with dew point included to calculate the DA?

Short answer: water vapor is less dense than air. Replace air molecules with water vapor and the density goes down. Refer to FAA-H-8083-25, Pilot’s Handbook of Aeronautical Knowledge p 4-5 for details

Look at the wet versus dry adiabatic lapse rate.

Density altitude is the air density expressed as an altitude above MSL.  Both temperature and humidity affect air density; however, temperature has a far greater impact.
Electronic E6Bs account for the effect of humidity by using both temperature and dew point temperature, the latter of which provides a measure of the air's water vapor content (technically it measures the temperature to which the air would need to be cooled for saturation to occur).
In contrast, the simple rule of thumb provided above only addresses the impact of non-standard temperature on density altitude. However, the effect of humidity can be "roughly" approximated by multiplying the dew point temperature (in degrees Celsius) by 20 and reading the results in feet.
This correction is then added to the above temperature correction rule of thumb for density altitude.  Note the correction for humidity is highly non-linear and so it provides only a rough approximation.  It should only be used in the dew point temperature range of 5-30 degrees Celsius.
Details can be found in Guinn, T. A., & Barry, R. J. (2016). Quantifying the Effects of Humidity on Density Altitude Calculations for Professional Aviation Education. International Journal of Aviation, Aeronautics, and Aerospace, 3(3). http://dx.doi.org/10.15394/ijaaa.2016.1124

Dew point is a measure of how saturated a given parcel of air. It is the temperature if the air is cooled to it, the air will turn to a liquid state. A simple way to imagine the dew point is the temperature at the base of a cloud.
Dew point isn't used in calculating density altitude because density altitude is pressure altitude, corrected for non-standard temperature. The temperature could remain the same, but pressure will change as fronts move over an airport, thus changing density altitude.
An excellent summary of dewpoint can be found in a 2003 article in AOPA from Thomas Horne, Dew Point Review (Internet Archive link).

The formula that you have listed is an approximation only.  The real formula from Wikipedia is:

Where the humidity factor comes in is the Molar mass of the air which is M in the equation.  Water has a smaller molecular mass then other components of air so it lowers the value of M and increases the density altitude.  Humidity is not usually included in density altitude calculations because calculating the Molar mass is difficult for a given humidity level.  However certain apps like ForeFlight are able to calculate the humidity effects of Density Altitude and can provide a more accurate density altitude then what you would calculate with pressure altitude and temperature alone.  Humidity is not as much a contributing factor than the Temperate and Pressure altitude but it is by no means negligible. For example, in ForeFlight I saw an airport where the pressure altitude and temperature was lower than standard; however, the density altitude ended up being 1000 feet above the airport elevation because the humidity level was 60%.


text:
How can a crew be warned of "Clear Air Turbulence" during the flight?
Is there any piece of equipment, on board, to assist the crew to identify and avoid clear air turbulence (CAT)?

Ahh. The holy grail. Following a huge amount of research over the years, LIDAR seems to be the most promising. See this article from Wired. Realistically, the best piece of equipment today is the VHF comm radio where the poor guy flying through it reports it to ATC and it gets passed to you.

Not in terms of sensor equipment. Weather charts obtained prior too flight identify areas of possible CAT. Usually, this data is not accurate enough to make changes to planned altitude/route prior to flight, but at least it increases awareness of what to expect. In flight, pilot reports from other aircraft, relayed by ATC, aid in selecting/changing altitude to an altitude where no turbulence, or less turbulence is expected.

I think there are experiments with airborne lidar goig on (e.g. DELICAT), else Lidar is used from the ground as CAT contains no water vapor, thus is not detectable using radar wavelengths. See: How does radar detect turbulence and wind speed?

Possible duplicate of How do pilots foresee turbulence?

I have asked about CAT, not turbulence.

I'm voting to leave open on the grounds that CAT isn't addressed much in the other question (and one of the answers that does address it isn't accurate.)

@eduardoguilherme The T in CAT stands for "Turbulence." You said so yourself in your question...

Not on board the aircraft as of yet, no (unless, as @Gerry said, you count the radio that lets you hear about the person ahead of you running into CAT).
There are ways of detecting CAT, however, which could become standard aircraft equipment in the future.  LIDAR is one possibility, as both @Gerry and @mins mentioned; see also here and here.  Interestingly, the idea of using LIDAR to detect clear-air turbulence has been around since at least the late 60s, but, when it was first proposed, they couldn't get the sensitivity high enough; later, when this obstacle was overcome, they faced a different problem, which was that the equipment required was far too heavy, bulky, and power-hungry to be practical, which remained the case until very recently.  N-slit interferometry can also be used to detect clear-air turbulence (see here, for instance), and an interferometric (I think; the patent description is remarkable in its unintelligibility) CAT detector was patented in 1978, but the equipment required is far too large to fit on an aircraft; as a result, most work with interferometric CAT detection seems to have been in the area of developing a ground-based clear-air turbulence detector that could monitor runway approaches for dangerous CAT.


text:
Is "laze" (lava + haze) as hazardous as volcanic ash to turbo engines?
Reading an article about Pahoa, I read that erupting Hawaiian volcanoes produce a combination of lava and haze - resulting in clouds that contain hydrochloric acid.
 Are these as dangerous as volcanic ash clouds?  What are the possible consequences for nearby flights?

One is chemically corrosive, one is mechanically corrosive. Flights are sure to be avoiding flying thru either.

Laze is produced by lava running into the ocean, which limits both volume and dispersion of harmful products. Volcanic ash can be blown high into the air in astonishing amounts by explosions at a volcano summit, making it possible for wind to spread the particles over a wide area.
The 2010 eruptions in Iceland blew ash several kilometers into the atmosphere and across much of Northern Europe, shutting down air traffic for days. Both types of emissions have abrasive particulates and corrosive chemicals, so the biggest determinants of harm are dispersion and volume. Ash is by far the more dangerous.
see Kileua sends ash 30,000 feet high and this isn't even considered a very explosive volcano.

I don't feel this answers the question. Why is it dangerous?

@Cloud Both types of emissions have abrasive particulates and corrosive chemicals, so the biggest determinants of harm are dispersion and volume.


text:
What “slang” do pilots and controllers use to refer to a “hard landing”?
Aviation is an environment which pilots and controllers use various phrases "off stage" to describe things in different terms from the standard ones. How do they say (far from the mics), HARD LANDING?

And in today's news

This answer is relevant.

Pilots often refer to hard landings using terminology usually associated with carrier landings, such as "welcome aboard" or "caught the 3-wire" or something similar.
One of the most memorable remarks made on a firm landing was something that a flight attendant said after I absolutely pounded one in on my first or second day in the left seat of the 737.  It was the last flight of the night, and things weren't working out right, and at about 20 or 30 feet, the check pilot (instructor) in the right seat just said "oh, boy" which you know isn't a good sign.  And he was right, I absolutely slammed the landing on, really one of my worst ever.  The only saving grace was, I was still monitoring the PA audio (which I normally wouldn't at that point, but my left-seat habit patterns were still evolving) and one of the flight attendants immediately got on the PA and said, "And that, ladies & gentlemen, is why we tell you to turn your electronics OFF before landing!"
Apparently they'd had somebody who simply wouldn't turn off his phone, wouldn't get off his laptop, and so on, and the FA figured it'd be worthwhile to reinforce a point (even if the one had nothing at all to do with the other in fact).  I've never kissed a flight attendant, but that night I just about could have.

I didn't see this myself, but one time a passenger asked the pilot after a very hard landing. 'did we land or were we shot down?'

At my gliding club, we sometimes witness a "five point landing": main wheel, nose wheel, tail wheel and both wingtips...

I remember a pilot once saying, right after one of those landings, something like "Ladies and gentleman, we have attacked Chicago's O'Hare Airport and it has surrendered!"

I thought the 3-wire was supposed to be the one you aim for?  The others are there in case you land a little early or late, but the 3-wire is supposedly the ideal landing on a carrier?

@DarrelHoffman Yeah, the point of the comment isn't that it was a bad carrier landing, but that it was a carrier landing -- and every bit as firm as is expected for such events.  (But yes, the 3-wire is the desired one to catch.)

@DarrelHoffman, not to be too pedantic, but you don't actually "aim" for a particular wire, you simply fly glideslope, line up and AoA all the way to touchdown.  If you are on-and-on at touchdown you should catch the 3 wire simply because the targeted hook touchdown point happens to be in very the middle of the 4 wires.  The 3 wire tends to take a beating as a result, and if they need to strip it mid-recovery they will adjust the glideslope to target the 2 wire.

Usually it's just "hard" prefixed by a word or phrase. He/she:
Arrived hard
Touched down hard
Landed hard
Set it down hard
Alighted hard (maybe for someone who likes to use fancy words)

I'll honestly say I can't recall ever hearing "alighted hard" used as a descriptor of a firm landing.  Or of anything else, as far as I can recall.  The rest I'd agree with, though. (Maybe if I flew with more Brits, I might have heard that one too.)

Yeah lol I was just reaching for as many words as I could think of.   Pounded it on on your answer is good. I used to help a structural engineer review FDR plots of CRJ hard landings sent to us by a Chinese operator. They were monitoring crews by downloading the FDR data daily because the crews never reported them, the system there being somewhat punitive.

In flight schools they will often refer to doing "slam & go's" as opposed to "touch & go's."

I have heard/used "Turned it into an RG" for fixed gear airplanes.

I’ve always called any airplane a student bounced five or six feet off the ground because it was stalled in from that height ‘pranged.’ Of course then the aircraft would probably ‘wheelbarrow’ for about 500 feet, with help from the student.
No idea where the saying ‘pranged’ came from. I was taught the term so I just passed it on.
Probably came from Yeager.
CFII/MEI

When I trained in the US, a hard landing was usually met with a call from the controller to "Taxi what is left of you're aircraft back to the ramp". No such good quality banter have I ever heard over in the UK unfortunately.
It did make me chuckle.


text:
Could Concorde have used swing wings?
Would it have been possible to equip the Concorde with swing-wings to help with lift at lower speeds? The delta wings on the aircraft worked well at high speeds, but it took a long time for it to achieve takeoff speed. A swing-wing would give it the aspect ratio for low speeds and delta form for the high speeds.

Actually, the Boeing design for the SST (Super Sonic Transport), the Boeing 2707 had a swing wing (swing wing was the hot thing during that time). It is instructive to look at what happened to that design.

Image from up-ship.com
The 2707 concept was a larger aircraft, with a 174' wingspan for TO/landing and low speed regime (In comparison, Concorde had a 84' wingspan and carried half the passengers). While the wings were swept back, the wing area increased from 1828 $m^{2}$ to 2743 $m^{2}$.
The engines were carried on the stabilizers. During the development Boeing had significant problems associated with the swing wing mechanism. Concerns about stability and payload capability lead the design to be lengthened and added canard to the front to meet rotation requirements, though the new design 2707-200 kept the swing wings.

Image from up-ship.com
However, these changes did little to solve the problems. When kept near the centerline, the hinge mechanism interfered with the undercarriage. The hinge mechanism was so heavy that it negated the advantages of swing wing, leading to a very poor payload to weight ratio. Studies indicated that the aircraft would run out of fuel halfway across the Atlantic, essentially killing the project.
Due to this, Boeing decided to use a fixed wing platform in its next iteration, the 2707-300, which looked quite similar to Concorde.

"Boeing 2707-300 3-view" by Nubifer - Own work. Licensed under CC BY-SA 3.0 via Commons.

While the wings were swept back, the wing area increased from 1828 m2 to 2743 m2
Why does swinging the wings backward increase the area?

Well, it would help with low speed lift, but it would have the following problems:

The fuel tanks in the wings would have to be rearranged significantly affecting the CG, CL, and CP. As seen in the diagram below, the fuel tanks fill most of the wing, so the wing at low-speed mode would create stresses on the airframe as well.

The engines would also have to be relocated to another position, which without even more modifications to this airframe could not have been done.
The swinging mechanism for the wing would have taken up much room because it had to move a wing of such size.

So, if the Concorde would have had swing-wings, it would have been a radically different airframe. However, because of the difficulty of making a swing-wing mechanism for such a large aircraft (Existing swing-wing aircraft are either small compared to airliners or have plenty of room for the mechanism) and the many more forces involved with this, it would not have been a good idea.
Diagram and source here

"Existing swing-wing aircraft are relatively small compared to passenger airliners" - I don't agree, the MTOW of the Rockwell B-1 Lancer (swing-wing) is higher than that of the Concorde and the A321-200, to name a few

@ROIMaison Will change that

authors: Даниил Галахов


text:
Which are the largest planes to have operated on an aircraft carrier?
In 1963, the C-130 was tested by the US Navy for air carrier operations. Have there been any other comparable or larger aircraft that have landed and taken off from the deck of an aircraft carrier?
By large, I am referring to two parameters: wingspan and weight.

U-2s also landed and took off from aircraft carriers.

U-2 is neither heavier nor has wider wingspan than C-130

@JanHudec "...Have there been any other comparable or larger aircraft..." ;-)

The KC-130F that performed the trials from the USS Forrestal in 1963 appears to be the largest, I know of nothing else of that size, weight and span that has done it.

And no arresting hook, as seen in the U-2 video link above

The largest aircraft by weight and wingspan is the C-130, and then the U-2 by wingspan and the A-3 Skywarrior by weight. The C-130 was only used for testing to see if it would be possible to use it as a carrier-onboard-delivery (COD) aircraft. The U-2 had one operation mission, but otherwise it was only used to evaluate the possibility of operating it from carriers in order to reach otherwise inaccessible places.
The A-3 is the only one of these to operate routinely from an aircraft carrier, and did so from 1956 until it's retirement (EA-3B) in 1991.

EA-3B, "Whale" the Skywarrier.

That's the largest aircraft that did regular operations from carriers. The Hercules is larger, but was only used for trials on an empty deck.


text:
What is the fastest accelerating aircraft on takeoff?
As of Feburary 2017 the Tesla Model S has the fastest 0-60mph time of any production car. With the fastest 0-60mph time recorded as 2.28 seconds.
This got me thinking, is there any aircraft that whilst on the ground can achieve a similar 0-60mph time?

0-160mph in 2 seconds for a carrier launch.

I definitely had carrier launches in mind, but that doesn't really count as the aircraft isn't producing all of the acceleration.

Then no.  0-150 yes, but any reasonable performance car will beat an aircraft 0-60.  After that the story changes.

A glider in a winch launch isn't much different from a carrier launch and faster at 60 mph than any Tesla.

Actually I think the fastest 0-60 might be RPG or AT missles because it need to accelerate to speed, usually directly to cruise speed, within the length of the launcher tube. And surely it is an aircraft.

@user3528438: If you're going to allow those, why not the self-guided projectiles launched from a rail gun?  0-Mach 7 in a few dozen feet :-)

@jamesqf Because if it flies ballisticly it's not  an aircraft anymore.

Airplane performance really isn't gauged that way, partly because quick acceleration 'drag race' data is not a pertinent aspect of their flight envelope.  That being said, there are a few examples, one being from the TV show Top Gear where a Bugatti Veyron races a Eurofighter Typhoon - and gets smoked by the jet.

I don't have data on jet fighters as far as linear acceleration; a quick calculation could reveal some basic data.
Take the case of an F-22.  Assuming a 55,000 lbf takeoff weight, lineup, 70,000lbf thrust at full power, then brakes off and rolling, we expect to see an acceleration around:
$$(70/55)*32.2 ft/s^2 = 41 ft/s^2$$
This also assumes no friction from the wheels or air resistance during the takeoff roll.
By basic kinematics, the jet would reach 60mph or 88ft/sec in 88/41 = 2.15 seconds.
So could an F-22 or another high performance jet fighter beat an Model S in a drag race?  Well it's theoretically possible and and as the above example shows, it has happened and it didn't end well for the auto.
As for airliners, the car is going to smoke them until the car reaches Vr for the jet or thereabouts.
And there is no way in hell a Model S is going to do 0-1000 mph.

It would be interesting to find out how much of a difference taking into account frictional forces would make.

Well, if the friction is 0, the aircraft will have the highest acceleration, whereas the car will just sit there spinning its wheels and not moving at all.

@JörgWMittag if you go that far, Aircraft will get no acceleration either, because there is not thrust without friction/viscosity... To get the acceleration for both, just make the integral of power (~constant), this give speed as a square root of time. More power/weight = win

Ground takeoff isn't ideal because the jet need time to ramp up power. If some kind of "wheel blocker" is used, like ones on ski-jump carriers, the jet would perform better.

55/70 is less than 1. How can 32.2 become 41 after multiplication by a factor < 1?

What about the difference between AFTERBURNERS ON and AFTERBURNERS OFF ?

Would the jet engine be able to produce full power at takeoff?  Doesn't thrust depend on airflow, with some of that being produced by the ram effect of the aircraft's speed?

Koyovis, point noted and updated.

@CarloFelicione Yes but I reckon it should be 70/55 * 32.2

Where's the 32.2 constant come from?

"Well, if the friction is 0, the aircraft will have the highest acceleration, whereas the car will just sit there spinning its wheels and not moving at all."  Tell that to the folks who think an airplane couldn't takeoff from a treadmill!

That would be the aircraft with the highest thrust-to-weight ratio. This site lists the fighters of the world with a bit of a puzzling caveat for TO weight:

TWR or T/W ratio = (Max Thrust of Engine / (Empty Weight + (3.505 Tonnes of Fuel & Weapons, or only Internal Fuel)))

1.30 - Su-35BM
1.29 - F-15K
1.26 - Su-27S
1.25 - Eurofighter
1.24 - Mig-35
1.23 - Su-27SK & J-11A
1.19 - Rafale C
1.19 - Mig-29M/M2
1.19 - F-15C
1.18 - F-22 (T/W = 1.37 with Round nozzles)

Let's take the F-22 with the round nozzles, whatever those may be: thrust-to-weight of 1.37. Weight W = m * g, so
$$ a = \frac{T}{W} \cdot g = 1.37 * 9.81 = 13.44 ~\text{m/s}^2$$
60 mph = 26.82 m/s, and with V = a * t we get for t = V/a = 2.0 sec. The F-22 with the round nozzles reaches 60 mph in 2.0 seconds.
Or with TWR the thrust-to-weight ratio, and all SI units: $$ t = \frac{V}{g \cdot TWR}$$

Edit
The above is of course for frictionless circumstances, as @Manu H pointed out in a comment. Also, @Zeus pointed out that the "F-22 with the round nozzles" never became anything more than a gleam in some engineer's eye. So what do we get if we take the aircraft listed as highest T/W - the Su-35 - and make an attempt at a ROM for friction effects?
Let's take an SU-35 at TWR of 1.3. Resisting friction is caused by:

Rolling friction of the tyres. This Wikipedia site lists rolling resistance coefficients, let's take 0.006 (about the minimal car tyre value), so tyre drag = 0.006 the weight of the aircraft.
Aerodynamic friction. Let's assume that this is caused by lift-less drag $C_{D_0}$ only. This Wiki site gives subsonic drag coefficient of 0.021 for a Phantom, let's take that. This Wiki site gives further data on the Su35, such as max thrust and wing area. In order to match the TWR of 1.3 with the stated afterburner thrust of 284 kN, we'll take a mass of 22,269 kg.

The aerodynamic drag is a quadratic function of the velocity. At 60 mph = 26.82 m/s, the aerodynamic drag = $ C_{D_0} \cdot \frac{1}{2}\rho \cdot V^2 \cdot S$ = 0.021 * 0.5 * 1.225 * 62 * 26.82$^2$ = 574 N = 0.2% of thrust. That is at the end speed, the average weighted value is a third of that = 0.07% of thrust
Tyre drag = 0.006 * m * g = 1,311 N = 0.5% of thrust
So if we account for drag, we need to use about 99.4% of thrust. Time to reach 60 mph now becomes
$$ t = \frac{V}{g \cdot 0.994 \cdot TWR} = \frac {26.82}{9.81 \cdot 0.994 \cdot 1.3} = 2.1 ~\text{sec}$$
What a way to spend a Sunday...

you should mention this is only theorical calulation not taking into account several factors such as drag.

Or limited airflow at low speed

If we are not limited to 'production' aircraft, then P-42 (a stripped-down Su-27 that set the world climb record) had TWR (in static conditions) in excess of 2. (It couldn't stand on the brakes at full power and had to be tied). This will easily win over Tesla, but then it would be fairer to compare it to dragsters. If we only account for production models, then we can't use "F-22 with the round nozzles"...

@Zeus Oh right-o, I was wondering what that was.

The fastest acceleration of an aircraft from a stationary position is found on catapult launched naval aircraft. For one like the FA-18, it goes 0-165mph in two seconds, which exceeds the Tesla figure by a wide margin.
Of course, that's with some assistance from the catapult.

The fastest self-accelerating airplane ever built as far as I know was the Sukhoi π-42, a modified Cy-27. Its FAI recorded takeoff weight was 14,100 kg, and its thrust was recorded as 2x13,600 kg, giving nominal T:W at takeoff of 1.93, giving 0-60 mph in 1.41s but this is kinda theoretical: most fighter engines produce less than their rated thrust while the aircraft is static, and substantially more at higher indicated speed. Based on scaling the performance of the Su-27 in DCS (18,000 kg takeoff weight, 2 x 12500 kg thrust), these nominal numbers give the π-42 a likely 0-60 mph time of 1.8 s. Based on the same testing and assuming the same SFC, The 3000m climb woukd have required about 500 kg of fuel, and in practice there's probably some margin on that number, so a min fuel  π-42 might be able to squeeze under 13,600 kg and over 2.00 nominal T:W for a 1.75 second 0-60.


text:
Under what circumstances does a flying club need to do 100-hour inspections?
I am part of a not-for-profit 501(c)7 flying club that owns a handful of airplanes. These airplanes may be used only by the club members. Members pay monthly membership dues; and pay for Tach time to use the airplanes.
Are our aircraft subject to the requirement of 100-hour inspections?
Possible contributing factors:

Some members are flight instructors, who offer flight instruction in these airplanes.
The students (Receiving dual instruction) may or may not be members themeselves.
A club rule says that only approved club flight instructors may offer instruction in club aircraft.

Does one of these bullet points trigger a requirement for 100-hour inspections?
The relevant FAR is 91.409, which says in part "no person may give flight instruction for hire in an aircraft which that person provides, unless within the preceding 100 hours of time in service the aircraft has received an annual or 100-hour inspection".
If we required student pilots to become members of the club prior to receiving any instruction, would we no longer be subject to requiring 100-hour inspections?

I don't think [tag:currency] applies here. I edited to replace it with some (seems to me) better tags; by all means feel free to [Edit] further.

AOPA addresses this very matter in their question of the month found here

A flying club aircraft normally would not need to receive 100-hour
  inspections.  FAR 91.409b requires a 100-hour inspection when a person
  receives instruction from a flight instructor who also provides the
  aircraft.  The FAA guidance further clarifies that the 100-hour is
  required when a person is providing flight instruction for hire in the
  instructor’s personal aircraft.

Your issues arise with these two points

The students (Receiving dual instruction) may or may not be members themeselves.
A club rule says that only approved club flight instructors may offer instruction in club aircraft.

As the answer goes on to state

Some clubs require a flight instructor to be a member prior to giving
  instruction in club aircraft.  In this case, the FAA doesn’t view this
  as an issue as long as three conditions are met:

There must be no contractual relationship between the member-instructor and the flying club for the provision of flight
  instruction.
The member-instructor is not recommended or given preference by the flying club.
The club members are free to choose instructors who are not club members.

If any one of these conditions is not met, then a 100-hour inspection
  would be required for the club aircraft.

It seems we run afoul of this: "The club members are free to choose instructors who are not club members." Naturally we don't want to allow non-members to act as PIC in club aircraft.

Wait, isn't this totally self-contradictory? "Some clubs require a flight instructor to be a member prior to giving instruction in club aircraft. In this case, the FAA doesn’t view this as an issue as long as three conditions are met: The club members are free to choose instructors who are not club members."


text:
How is a variable sweep wing constructed?
How is a variable sweep wing able to change its sweep?

This is a pretty broad topic covering both aerodynamics and structural mechanics.

I edited the question to focus on what seems to be the main question, feel free to correct it or roll back if this isn't right. We already have a question on the aerodynamic aspects so a question on the structural aspects seems to be a good complement.

I vote to reopen as edited by fooot

It's about a huge hinge (source: This question).

The easiest way to sweep a wing is an oblique flying wing. This is essentially a straight wing with pivoting rudders and engine pylons which flies at an increasing angle of sideslip where a swing wing aircraft would fold its wings back.

Oblique flying wing (picture source)
If you want to know more, I suggest to read this website. Excerpt:

Although the oblique flying wing concept was described by Jones in
  1958 and a drawing by G. H. Lee was published in 1962, the concept
  received little attention until the 1990's when active digital flight
  control systems became more capable and widespread. The oblique
  all-wing configuration was studied in detail by researchers at NASA,
  Stanford University, McDonnell-Douglas, and Boeing in the 1990's as
  part of a NASA program.

If the wing is attached to a fuselage, only the wing should move. Again, this can be achieved by pivoting a straight wing, as done in the NASA AD-1 research aircraft, or by folding the wings back with a huge hinge at the wing root, as done on swing wing aircraft.

NASA AD-1 in flight (picture source)
The first aircraft with a variable wing sweep was the Messerschmitt P-1101 which was almost completed in 1945 when WW II ended. It was supposed to fly as a fighter-bomber with reduced wing sweep and as a pure fighter with increased wing sweep, and sweep could only be changed on the ground. On the basis of this design Bell built the X-5 research aircraft, the first to change sweep in flight.

P-1101 under construction (picture source)
The mechanics of a swing wing is rather straightforward: All you need is a big hinge connecting the wing spar at the root and a big hydraulic cylinder or spindle drive for adjusting the wing's sweep angle. And a flexible fairing at the wing root. And something to compensate for the shifting aerodynamic center.

Swing wing mechanism of the F-14 (picture source)


text:
When is a 100-hour inspection required if alternating commercial and private use?
First, a setup rehashing common knowledge: A plane owner, who's also a CFI, gets an annual done on his Cherokee and teaches out of it for 100 hours in three months. He flies it to a repair shop 3 hours away for a 100-hour check. After the 100 hour check is complete, he can legally use it to teach for 97 hours before the next 100 hour is due (if he doesn't reach a required annual first). This is because there's a 10 hour window past the 100 hour limit to fly a plane to a place where it can receive its 100 hour service, but that window doesn't add to the time of the next 100 hour time requirement for commercial use.
Now to my questions. In the above scenario, does the 10 hour window even apply to the owner? He's flying his plane non-commercial, not for hire, so in that capacity he should be able to fly it as long as he likes (until the next annual is due, of course).
What if the owner in the above scenario teaches out of his plane for 100 hours after the annual, stops teaching and flies his plane personally for 50 hours, then decides he wants to teach in it again? Can he get a 100 hour check to get the plane legal for commercial use? Is the next 100 hour due in another 50 hours? What if instead he flies it personally for 100 hours after the 100 hour is due? Does that mean he has to get two 100 hour checks done in a row to use it commercially again? Or does he have to get a full annual to reset the 100 hour clock?

This FAA legal interpretation covers a lot of scenarios and it looks like it answers yours too. Although it refers to rental, not ownership, the result is the same because rental time isn't instruction or carriage for hire.
First, what the regulation says (14 CFR 91.409(b)):

Except as provided in paragraph (c) of this section, no person may
  operate an aircraft carrying any person (other than a crewmember) for
  hire, and no person may give flight instruction for hire in an
  aircraft which that person provides, unless within the preceding 100
  hours of time in service the aircraft has received an annual or
  100-hour inspection and been approved for return to service in
  accordance with part 43 of this chapter or has received an inspection
  for the issuance of an airworthiness certificate in accordance with
  part 21 of this chapter. The 100-hour limitation may be exceeded by
  not more than 10 hours while en route to reach a place where the
  inspection can be done. The excess time used to reach a place where
  the inspection can be done must be included in computing the next 100
  hours of time in service.

The key phrase from the interpretation is this one (my emphasis), which really just restates the regulation in a simpler way:

if [you are] going to operate the aircraft to provide
  flight instruction for hire, then the aircraft must have had an annual
  or 100-hour maintenance inspection within the preceding 100 hours of
  time in service.

Now we can compare your scenarios to the ones in the interpretation. I won't quote the document here, but scenarios 1 and 6 together cover your cases.
In the above scenario, does the 10 hour window even apply to the owner? He's flying his plane solo, not for hire, so in that capacity he should be able to fly it as long as he likes.
Correct. If the aircraft isn't going to be used to do any paid instruction or carry paying passengers then he can fly the aircraft himself, rent it out, lend it to a friend etc.
What if the owner in the above scenario teaches out of his plane for 100 hours after the annual, stops teaching and flies his plane personally for 50 hours, then decides he wants to teach in it again? Can he get a 100 hour check to get the plane legal for commercial use?
Yes, in fact he must. The aircraft requires a check within the last 100 hours of operation to be legal for flight instruction for hire, or carrying paying passengers.
Is the next 100 hour due in another 50 hours? What if instead he flies it personally for 100 hours after the 100 hour is due? Does that mean he has to get two 100 hour checks done in a row to use it commercially again? Or does he have to get a full annual to reset the 100 hour clock?
The 100 hour clock is reset by a 100 hour inspection or by an annual, he doesn't need a second check. But note that if the CFI does get an annual, he still has to deduct any 'overtime' from the 100 hours:

You may perform an annual inspection rather than a 100-hour
  maintenance inspection when the aircraft reaches the 100-hour
  limitation under§ 91.409(b). However, if you perform an annual
  inspection on an aircraft that has exceeded the 100-hour limitation,
  you are still required to subtract the excess time from the next 100
  hours of time in service.

As for going over the 10-hour excess time, the interpretation covers an equivalent scenario (scenario 6) where the aircraft is flown to 112 hours legally by renters, inspected, and then used for paid training again. The explanation is poorly written in my opinion, but it confirms that the aircraft needs an inspection before any paid training can take place - which makes sense - and the next inspection after that must be made within 88 hours - which makes less sense.
Extending that forward, it would mean that if you fly to 199 hours, you need an inspection - either 100 hour or annual - to make the aircraft legal for paid training and the next inspection would then be due in 1 hour! I don't know if that's really the intended result of the interpretation because it seems to conflict with the regulation's wording about "within the preceding 100 hours of time in service". And what happens if you rent the aircraft out for 299 hours before using it again for training?
Maybe I'm just misreading or misunderstanding things completely, but it looks like the interpretation leaves a couple of loose ends and the only definite, official way to get them answered would be to ask for another interpretation.

Thanks. I still see an inconsistency between "The 100 hour clock is reset by a 100 hour inspection or by an annual" and "he still has to deduct any 'overtime' from the 100 hours" though. If you have flown the plane for personal use 99 hours past the 100 hour mark from the previous check, that means you have 99 hours of 'overtime' right? So if you then got a 100 hour inspection, and deduct those 99 hours of 'overtime', your next 100-hour inspection would be due in 1 hour of flight time.

I second Dazz M on that. It sounds like if the plane has been flown for 200 hours non-commercial, then you'd have to get two 100-hour inspections done back to back without flying in between before you can use it commercially. That doesn't seem to make sense. Take it to maintenance and say, "inspect it twice?"

@DazzM Good point, and I updated my answer. The interpretation does seem to address that scenario but unless I've missed or misunderstood something - which is entirely possible! - it's confusing and the conclusion is unclear (to me).


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Why do we always use circular nozzles for commercial aircraft engines?
Why do we always use circular nozzles for commercial aircraft engines? Other shapes like rectangular nozzles are used in military aircraft, e.g., Raptor, Nighthawk.

Most military aircraft use round nozzles too. As far as I can tell, B-2 Spirit, F-117 Nighthawk and F-22 Raptor are the only three (manned, production; there are probably some prototypes and there are probably some UAVs) aircraft in the world that use non-circular nozzles. So you should really be asking the other way around: why those three differ.

Simple geometry: Circles are strong, and provide a large area for the amount of material used. That means you get more intake/nozzle for the cost of the materials, while giving you a strong and (relatively) light engine.
A circle also helps to keep the airflow smooth, because it's (again, relatively) uniform: no weird edges, corners, or angled things to cause turbulence, vortices, or otherwise disturb or make the airflow "complicated". This is ideal at subsonic speeds.
Overall, this makes a circular engine the most efficient, and when it comes to commercial aircraft efficiency is king: the lighter and simpler you can make a safe engine, the better.
In some aircraft we will see "off-circle" (eg 737 engines, which have a not-quite-circular intake) or oval intake, but they're basically just circles where the dimensions have been adjusted to take a slight efficiency hit for the purpose of fitting the shape of the aircraft better.
Military aircraft which use rectangular air intakes tend to do so for two different reasons, rather than because it's actually any more efficient.

Stealth. Circles scatter radar waves in a "bad" way for stealth. Angles are much better for stealthy aircraft
Because that air intake is actually an Intake Ramp designed to slow the airflow into the engine by deliberately creating a shockwave to slow the air down. This is because jet engines need subsonic air to work properly: this is why you tend to see square intakes on supersonic aircraft. Commercial aircraft don't usually fly supersonic, though, with the notable exceptions of Concorde and the Tu-144 which both had square intakes for this reasons.

If you don't care about subsonic efficiency (Concorde/Tu-144, which were both designed to spend most of their working lives supersonic), or if you just want to be able to go supersonic at all, a square intake can make sense - but for those of us living life under Mach 1, and for airlines who care about fuel efficiency above pretty much anything else, a circle is best.


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Is it legal to fly GA aircraft in temperatures above what is shown on the performance charts?
Most Cessna aircraft have performance charts the range from 0 to 40 Celsius. I've had a CFI tell me that the FARs prohibit flight if the OAT exceeds 40C or whatever is shown on the performance charts. Is that a true statement? I have not found anything in Part 91 that addresses this.

Related high temperature questions for (mostly) part 121, and effects on performance, but nothing definitive to answer this question.

Did you ask him if all those Cessnas in Alaska are flying illegally because the chart doesn't show anything below zero?

Very, VERY closely related question:  Is there a maximum airfield elevation in which a Cessna 172P can operate?

I cannot find any FAR regulation that prohibits flight outside of performance charts. Another point against this is demonstrated cross-wind, you can land (legally) when the cross wind is greater than the demonstrated cross wind in the performance chart. In fact, the manual states that this is not a limitation...

@RonBeyer That value is specifically called the Max Demonstrated Cross-Wind rather than Max Certified Cross Wind Limitation (which some aircraft do have) because it isn't meant to be a limitation. Most other things in the AFM don't have that distinction.

@Lnafziger I guess my point was its part of the published performance data. Even aircraft with a certified cross wind limitation are legal (but not smart) to fly outside of those limitations. Usually the only action in those cases is a note in the NTSB accident report about the pilot exceeding the certified limitations, but the FAA isn't issuing citations.

@RonBeyer I'm sure it does happen, but when/why would it be legal to operate beyond a limitation given in the POH? 91.9 is clear that pilots must comply with any stated "operating limitations".

@Pondlife Maybe the performance charts aren't considered "limitations"? You can't suggest that it is illegal for an aircraft to depart when the outside air temperature is less than 0°C? If that were the case, GA operations would cease in colder parts of the earth during winter months, and FAA officers would have a field day at the local FBO...

For Cessna's, Section 2 of the POH contains the operating limitations, FAR 91.9 prohibits operation outside of those limits. If it is not stated as a limitation in the POH, then it shouldn't be illegal to operate outside those ranges. I've read a couple POH's for 172's and I can't see anything that prohibits operation outside of a certain temperature range. The manual suggests that you shouldn't operate more than 23°C above standard temperature, but doesn't state it as a limitation.

@RonBeyer Yes, and that was exactly the point of my comment above about Alaska: clearly the fact that no data exists is not by itself a limitation. However, your response to Lnafziger on a different point was that it's legal to "fly outside" a specific, certified limitation of the aircraft, and I don't see how that can be possible per 91.9.

@Pondlife Yes, you're right, I didn't get that part right.

Thanks to everyone who responded to this question, it's become a real "hot topic" with our flying club. I live in Las Vegas where the temps in the summer often exceed 40c. I called Cessna and the person I spoke with (took awhile to get to the right one) stated that: " The performance charts are not l;imitations, if it was a limitation it would be in section 2 of the POH". I always try to avoid the miserable experience of flying in 104+ temps, but now I'm more confident that I'm not violating the regs.

Yes, it is legal, disregarding any catch-alls such as 91.13 reckless operation or ones pertaining to pre-flight planning data like takeoff distance.
It is simply left to the pilot's good judgement for a go/no go decision depending on whether or not the flight can be completed safely.
Edit: Re-reading this answer much later, I would like to emphasize, "disregarding any catch-alls".
With regard to the catch-alls such as 91.13, this could very easily be considered illegal, but no rule with a statement clearly identifying taking off with temperatures higher than those on the charts as a prohibited action exists to my knowledge.  Do not do it.

What do you base this legal opinion on?  If you do not have performance numbers, how do you know that you can safely takeoff and climb under those conditions?

You're shrugging off 91.13 as if taking off in uncertain conditions wouldn't constitute "reckless operation".  I would argue that taking off when you can't be certain of your ability to climb would be quite reckless, and 91.13 may likely cover it.  It would be good if you could cite something to prove otherwise.

@Lnafziger me?  I wouldn't.  Somone with decades of experience and thousands of hours in type could make a determination based on experience.  How far over the limits?  1 degree?  2?  50?

@JayCarr no, I'm not, and that's why I mentioned it in the answer.  Shrugging it off would be to completely ignore it and not include it here.  However, I understood what the asker is really asking, which is whether there is a rule that specifically addresses this particular situation, taking off at a temperature above those shown on the aircraft's charts, directly, which 91.13 does not.  Hence the phrase, "disregarding any catch-alls such as 91.13."

It is legal, though not recommended as aircraft performance has not been fully quantified in this regime of the flight envelope.  As pilot in command, you have the final authority to decide to proceed here.  Keep in mind, however you could be held civilly liable should an accident occur under these conditions.

Possibly not. The big problem here is 14 CFR 91.103(b)(1):

§ 91.103 Preflight action.
Each pilot in command shall, before beginning a flight, become
familiar with all available information concerning that flight. This
information must include -
(a) For a flight under IFR or a flight not in the vicinity of an
airport, weather reports and forecasts, fuel requirements,
alternatives available if the planned flight cannot be completed, and
any known traffic delays of which the pilot in command has been
advised by ATC;
(b) For any flight, runway lengths at airports of intended use, and
the following takeoff and landing distance information:
(1) For civil aircraft for which an approved Airplane or Rotorcraft
Flight Manual containing takeoff and landing distance data is
required, the takeoff and landing distance data contained therein; and
(2) For civil aircraft other than those specified in paragraph (b)(1)
of this section, other reliable information appropriate to the
aircraft, relating to aircraft performance under expected values of
airport elevation and runway slope, aircraft gross weight, and wind
and temperature.

Once you exceed the top of the performance charts (e.g. 40 ºC in a Cessna 172), you no longer have the required takeoff/landing distance data available to comply with 91.103.
Here's the takeoff data for a random Cessna 172. Notice that the notes provide a procedure for handling headwinds/tailwinds, as well as grass runways - but there's no procedure specified for temperature excursions.
If an FAA inspector asked you what your takeoff distance was on a 50 ºC day (and again, 91.103 requires that you determine this as part of your preflight), there'd be no way to authoritatively answer them.

This is a different story in aircraft that provide performance tables based on density altitude, as opposed to pressure altitude + temperature. You can compute the density altitude yourself for any temperature.
As pointed out in the comments below, one could argue that the FARs only require that you're familiar with the performance data. Whether that's referring to the performance data in aggregate, or the performance data that applies to your current flight conditions is perhaps a gray area. I don't believe the FAA has ever given more concrete guidance on this one.
(However, I have also seen flight schools take the opinion that operations above 40 ºC are not authorized for the above reasons.)

For which civil aircraft is an "approved flight manual containing T/O and landing distance data" required? Does that refer to all civil aircraft or only aircraft carrying passengers for hire, for example? I have no idea if this is the case, but I could imagine it being true that Cessna does not have to publish this data, and the fact that they do does not mean you must stay within the described bounds.

@randomhead The general requirement for an "approved Airplane Flight Manual" comes from the airworthiness standards, specifically 14 CFR 21.5. More specifically, the content of the AFM for a modern, normal category aircraft is specified 14 CFR 23.2620. (Aircraft built before March 1979 did not have to have an AFM. In that case the format was less standardized, but you'd still have a POH and/or placards. Hence the alternative requirement for "reliable information appropriate to the aircraft" in (b)(2).)

Hm, okay. But going back and looking at the actual wording, all it says is that the pilot must, before flight, become familiar with the T/O and landing distance data contained in the AFM. Okay, so it's 44 out, or M14, and the data is no longer relevant—but the FAR doesn't say the pilot has to be familiar with the relevant data, it says they have to be familiar with whatever data happens to be in the AFM. See the loophole?

The bottom line is whether what you're doing is reasonable.  If your chart tops out at a 5,000 foot takeoff distance, and the temp is 1 degree higher than the limit of the chart, but you are using a 13,000 foot runway, I think you could very strongly assert that the takeoff distance would be within the safety margin.  I recognize that's an extreme example, but you probably see my point.

@RyanMortensen While you're almost certainly correct about the safety margin, the question isn't what's reasonable, it's about what's legal.


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Does windmill restart often work for airliner engines?
There seems to be a certain class of airline accident where an airliner engine flames out followed by further unfortunate events. Very often, pilots are recorded as having attempted to restart a failed engine by windmilling, which seems to involve attempting to restart an engine from the rotation due to the passage of air due to airspeed. In accident reports this rarely ever succeeds, leaving the impression of a rather worthless exercise.
But, of course, there's a massive bias here because these are the reports of accidents and if the windmill start worked, things wouldn't have deteriorated.
So, in rough terms, how desperate is attempting a windmill start. Is it in the land-on-water level of optimism, or something which usually succeeds which we never hear about?

As you observe, there's a huge selection bias, there.

Windmill starts are a rather last-resort type of thing. Lots of things have to go wrong before pilots will be attempting that.
First, an engine has to fail. Since airliners are required to have at least 2 engines, and be able to fly on one engine, this isn't a dire emergency. Pilots will work the associated checklists. If there is no apparent issue they may attempt to restart the engine using bleed air from the running engine. If there is an issue they will leave it shut down. With thousands of planes in the air, this sort of thing happens pretty much daily.
The chances of the other engine failing are very low, and generally due to some common circumstances that affect both engines. Pilots may be more aggressive in trying to get even a damaged engine started, as a damaged one is better then none at all.
The APU may be started and used to help start the engines as well. But this takes time, which tends to be running out when gliding with no engines running.
As ymb1 pointed out, engines are required as part of certification to demonstrate windmill starts. Issues that can prevent this from working tend to get the attention of the safety folks. This establishes limits and guidelines that are put into the aircraft checklists. Pilots know that if they maintain a certain air speed, the engines should reach a certain RPM, which will allow them to be started. Of course if multiple engines have already failed, there is a chance that this issue will prevent a restart.

As Oscar Wilde said, "pure and simple truth" is rarely pure and never simple...
The "not-so-simple" part is that engines don't just flame out for no reason. They only flame out because something is abnormal, either with the engine itself or with the environment (ash clouds, sandstorms, torrential rain, fuel supply, etc).
Therefore, the probability of a successful engine restart is already compromised in some unknown manner by the fact that it stopped.
Deliberately shutting down an undamaged, low-life engine and demonstrating that it will relight, as part of the flight test certification program, is about as good a test as is practicable (there is no way any flight test program will go around looking for volcanic ash clouds to fly through, water ingestion tests done on an engine on a test stand on the ground can't be followed by a windmill relight test, etc) but it's not necessarily completely representative of a real-life engine relight situation.

"water ingestion tests done on an engine on a test stand on the ground can't be followed by a windmill relight test" why not?

@Random832 Because the engine is stationary on the ground so you can't windmill it, unless you have a really big hairdryer to push 550mph air through it.

@DavidRicherby Like the fan on another jet engine?

@immibis No. How would you couple them together? How would you deal with the high velocity of the exhaust gasses and their lack of oxygen? Their turbulence? Their temperature?

@DavidRicherby You can't do the test in a wind tunnel?

@Random832 Wind tunnels can cope with having that much water dumped into them?

In actual accidents pilots confusing one engine for another seems to be a relatively common occurrence. I'm not an accident investigator but I seem to remember when I was growing up there were at least two major disasters on UK soil (Kegworth and Manchester Airport) when that happened. OK, so Manchester was on the ground, but in a "oops, wrong engine" accident, presumably windmill starting an undamaged engine would have been useful?

Manchester Airport wasn't a wrong-engine accident; the pilots recognised that the left engine had failed and shut it down.  Unfortunately, by that point, the engine failure had already set the plane on fire.

The problem with windmill starts on a gas turbine is - it requires a fairly high airspeed, as mentioned in this bit on starting aircraft engines.
Typically, the APU, itself a smaller turboshaft engine, has an electric/battery start, while high pressure bleed air from it or another running engine or a ground supply is used to start a main engine.
If your only option is a windmill start, it means you have no bleed air from another running engine, or the APU, as in all of them have failed. Quite probably, you've run out of fuel, so a windmill start is a waste of time. APU's tend to start far more quickly than main engines, so if you don't have the time to start the APU, you probably don't have the altitude to swap for airspeed to try a windmill start.
The last thing you want to do if you're in that shape is lose more altitude than necessary when you have no power, and no other way to restart your engines.
Windmill starts on piston/prop planes in the 1930's and 1940's were performed, usually when the electric starter on one engine of a four engine aircraft had failed while the aircraft was parked. They would take off unladen on three engines, with the fourth prop feathered, and use the airflow to crank that engine to a start, then land and pick up whatever they were carrying and go on their way.
Or, WW2 fighter aircraft would windmill start when the pilot dropped external tanks but forgot to switch fuel supply to the main tanks. They'd just change the fuel settings, nose down and reset prop pitch, and fire the engine back up. In some cases, it was quicker than an electric start, or in the case of naval aircraft that used a high pressure shotgun shell starter, it was the only way to restart the engine in flight.


text:
Could a pick up truck save a plane with failed landing gear?
I recently saw this fictionalized video.

While it's not real, I do wonder if this could be possible, even if not the best way to handle the situation.

Related: How can I calculate the force on the nose gear at landing?

a bit heavier than the GA aircraft, a harrier once landed on a sturdy stool

Can't let this go by without a link to 405 The Movie.

For the Boeing 727 that you see there, that whole procedure is impossible with a pickup truck. Given that the 727 is operating at its Operating Empty Weight, 102,900 lb OEW for the 727-200 advanced, and the force on the nose gear is somewhere around 10% at rest, that would put it way over the limit for that particular truck model (Nissan Frontier at 4,690 to 5,816 lbs GVWR). That would be even less pretty for the truck since the force at touchdown on the nosegear should be higher.
Also, according to this thread, the maximum speed for that truck is about 95 mph. Assuming that this Yahoo Answers is correct, the landing speed of a 727 is 130 to 150 knots (150 to 170 mph). This is way greater than the speed of any pickup truck.
I had to make a response like this some time ago as well to the exact same video someone had a question about.
However, you could land a GA plane on a pickup truck if you had the right equipment.
Nitpick on the video: if the plane's nose gear didn't deploy all the way, then truck or no truck, it should have collapsed on impact.

Actually landing on the trailer of a moving pickup truck is pretty common for seaplanes without wheels when they need to be repaired, here is a video of one taking off: https://www.youtube.com/watch?v=-JDogTLtels

I have to find it but I'm pretty sure there is a company that does seaplane storage/maintenance that has trailer landings, at least for the smaller ones with a low landing speed. Either way its not always a port ramp either, float planes can land in grass too.

Done smoothly, the force transmitted to the nosewheel at nosewheel touchdown will gradually increase to equal that on the nosewheel at rest, but not exceed it significantly. Vehicles can typically withstand weights well beyond their GVWR, though double might be pushing it. Also, Vref is much higher than the nosewheel touchdown speed.

Depends.  A large aircraft, no.  A jetliner weighs well over 50,000 lbs on landing and wouldn't even fit on a flatbed.  CGI films like '405' are spectacular but unrealistic.
Now it is possible for a light airplane to do this, or potentially land the whole airplane on a moving truck with a very skilled pilot in the cockpit.  A friend of mine in Alabama does an air show act with this kind of stunt using a piper cub. See 13:00 into the video.

Granted Greg is a very senior air show pilot with more combined flight time than anyone reading this and the average GA pilot will probably not have the skill to pull this off successfully.
Landing gear problems, especially problems extending the nose wheel seem pretty hair raising but can usually resolve with a belly landing and relatively minor damage as demonstrated by this A320 making an emergency landing at LAX in 2005 after the nose wheel steering failed.

Even a complete failure to the landing gear to extend can be remedied with a belly landing on a smooth, long runway without nearby obstacles.  The jet will touch down most likely on the tail and engine nacelles, making the rupture of the wing box and fuel spillage and fire relatively low.  This is aided by crash and fire crews at the ready to foam down the airplane after it has stopped to keep the risk of an ignition source to a minimum.

It depends on the size of the plane. But in some cases yes. In that case the gear was not collapsed but since the relative speed of the truck and plane were the same it really does not matter if there was landing gear there other than to provide prop clearence .
You could of course attempt a maneuver like this.

I personally saw a truck - which had mattresses on the bed - accept the nose of a high wing twin engine turboprop with nose gear stuck up during landing and without incident! This happened one afternoon at Anchorage International Airport and many years ago! It was bigger than an Aerocommander but smaller than the commuter turboprops! The Anchorage tower might have a record of the incident?

Cool story, if only there was a reference?

Perhaps a Twin Otter? Anyway, I'd be surprised if that was actually a lower risk option than the aircraft simply landing unaided, nose to runway. Without incident, I'm sure it would have saved the plane a whole lot of repair expense, but it just seems like so much could go wrong.


text:
What are the services that MROs provide for corporate jets (private jets)?
What are typical Costs/Rates of Private(corporate jets) MRO Services in the Middle East/North Africa region?

Is there a particular country or area you are interested in?

yes MENA region !

In general, the MRO services for corporate jets includes, but is not limited to:

Aircraft major and minor inspections and overhaul
Aircraft defect rectification and trouble shooting
On-site defect rectification (away from base).
Compliance of regulatory directives.
Replacement of avionics units.
Aircraft engine maintenance.

The costs associated with these activities will vary depending on the model, location and aircraft condition. If you want actual costs, it would be best to contact the service provider directly.